LOS ANGELES — With the San Onofre nuclear power plant in California closed for good, there’s little doubt utility customers will see refunds for rates paid for the promise of power. The question is how much they could be.

Consumer groups and two Southern California utilities plan to meet behind closed doors Thursday to discuss an agreement on divvying up costs tied to the now-shuttered plant, which could top $3 billion.

Operator Southern California Edison and minority owner San Diego Gas & Electric Co. have been negotiating with consumer advocates over how to divide a long list of expenses from reactor equipment to replacement power that was purchased after San Onofre shut down.

The twin-domed, seaside plant between San Diego and Los Angeles was closed permanently by Edison last year after a long and costly fight over whether it was safe to restart. The plant hadn’t produced electricity since January 2012, after a small radiation leak led to the discovery of extensive damage to tubing that carried radioactive water.

The problems at San Onofre centered on steam generators that were installed during a $670 million overhaul in 2009 and 2010. After the plant was shut down, tests found some tubes in the generators were so badly eroded that they could fail and possibly release radiation, a stunning finding inside the nearly new equipment.

The California Public Utilities Commission has been overseeing a broad investigation into costs at the plant. SCE and the other parties in the talks asked an agency judge not to rule on key issues, pending the outcome of the settlement negotiations.

At issue has been who should take the financial hit — company shareholders or customers. In June, Edison chief executive Ted Craver pegged costs subject to possible refunds at roughly $1.3 billion, and the company has been taking steps anticipating such costs.

The Utility Reform Network, a consumer advocacy group involved in the talks, wants shareholders to bear significant responsibility for the shutdown, as well as substantial refunds for customers.